Conventional generation of computer graphics displays involves using geometric models to represent the real world. In a world made of geometric objects, these models would suffice for rendering realistic displays.
However, many natural phenomena are not effectively represented by geometric models. For example, although fog is comprised of tiny drops of water, using a model in which each drop must be individually placed is not efficient. Furthermore, a water drop model does not accurately represent our perception of fog, which is seen as a blur in the air and not as discrete drops. See Foley, et al. Computer Graphics (Addison-Wesley Publishing Company, Inc. 1990) p. 1011.
Particle systems are one approach to modeling objects that cannot be easily described with geometric models. A particle system is a collection of particles that evolves over time. Particle systems have been used to generate displays of objects such as fire, and distant landscapes with trees and grass. However, a shortcoming of these existing particle systems is that they are abstract renditions of fires, not based on physical reality. Also, they do not dynamically represent the behavior of a real fire.